A charged particle beam microscope, for example, a scanning electron microscope, is an instrument which irradiates a sample with a narrow electron beam, detects secondary electrons or reflected electrons generated by electron beam irradiation, and modulates the brightness according to the detection signal to form an image.
In the semiconductor manufacturing process, with the trend toward finer patterns, there is demand for an instrument which provides high measurement accuracy. As a dimension measuring tool for measuring the widths of fine patterns on the order of dozens of nanometers which can not be measured by an optical measuring instrument, a scanning electron microscope (length measurement SEM) for pattern width measurement which can take images of such patterns at a magnification ratio of 100,000 or more is employed.
In a scanning electron microscope used for this purpose, generally a plurality of frame images acquired by scanning an area including the pattern to be observed are added together to make an image for length measurement and the obtained image for length measurement is analyzed using various length measurement algorithms in order to compute pattern dimensions with high accuracy. Here, an image acquired by scanning the field of view for observation once is defined as a frame image and an image obtained by adding a plurality of frame images together is defined as an image for length measurement.
While the length measurement SEM is used for the purpose of measuring the dimensions of a pattern formed on a semiconductor substrate, it is not only used for that purpose but also in order to obtain pattern information about the materials and stereostructure by acquiring a plurality of images for length measurement and observing change of the acquired images over time. Among typical examples of pattern information which changes over time due to electron beam irradiation are shrinkage of resist patterns shrinkage and electrification of sample surfaces. The acquisition of pattern information using information about such change over time is used for measurement of the amount of resist shrinkage, Line-Space discrimination, and presumption of materials.
Photoresist for an ArF (wavelength 193 nm) exposure apparatus which has been used recently in the fabrication of semiconductor devices has a problem that it is damaged by electron beam irradiation, resulting in volume shrinkage. For a semiconductor device to demonstrate performance as designed, strict control of the shapes and dimensions of circuit patterns is needed in the semiconductor manufacturing process, but when a resist pattern is observed using a scanning electron microscope, shrinkage occurs and the original dimensions of the pattern could not be measured correctly.
In order to address this problem, according to Patent Literature 1, the magnification in the Y direction horizontal to a pattern is set lower than in the X direction vertical to the pattern to decrease the electron beam irradiation density to suppress shrinkage and compute pattern dimensions correctly. According to Patent Literature 2, even if shrinkage occurs, dimensions before shrinkage are computed by fitting an approximate function indicating transition of measured length values and shrinkage to experimental values.
Furthermore, since the base material of resist is made of insulator such as acrylic resin, a sample irradiated with an electron beam is charged, namely charge is accumulated on the sample surface. If charging occurs, shading contrast would change and it becomes impossible to discriminate whether the pattern being measured is a pattern to remain intact (Line pattern) or a portion to be lost (Space pattern). In order to address this problem, according to Patent Literature 3, the resist pattern outside and inside dimensions of two resist pattern images are measured and whether the resist pattern is a portion to remain intact or a portion to be lost is determined by comparison of length measurement values.
Furthermore, according to Patent Literature 4, an unknown substance can be identified by measuring the signal waveform of reflected electrons corresponding to the number of electron beam scans made until electrification on a substrate reaches an equilibrium state and comparing it with data on a known substance.